APLITES AND PEGMATITES 373 



collected of notable perfection. This flesh-red microcline was taken for 

 orthoclase in the field, and at first sight one would consider it a typical 

 case of the latter, but the moment flakes are tested with polarized light, 

 the peculiar twinning and extinction angles of microcline may be readily 

 recognized. The latter is about 15 degrees on the basal flakes and 5 de- 

 grees on the brachy-pinacoidal. The specific gravity, as determined on 

 pieces of from 15 to 20 grams, is 2.532 to 2.539 — values that are from 

 0.02 to 0.03 below the general average of microcline. 



A white feldspar is widely associated with the red microcline and is 

 of especial excellence in the Westerl}^ quarries. It is automorphic, but 

 must be dug out of the quartz that fills the interstices of the feldspar 

 crystals. The crystals are well developed and can be obtained at times 

 with fairly perfect terminations, although as a rule they have interfered 

 one with another in growth. The base is striated, so that in the field 

 they were all taken for albite. 



Optical tests of some cleavage flakes parallel with the base at once 

 revealed an extinction angle of zero degrees, while other flakes parallel 

 with the brachy-pinacoid gave one of 9 degrees. The specific gravity on 

 one lot was found to be 2.595, and on another 2.613. The latter value 

 approximates albite, but the optical properties were those of natron- 

 orthoclase. A positive bisectrix emerges nearly perpendicularly to the 

 brachy-pinacoid. The crystals exhibit, as determined by the eye alone, 

 P (001), M (010), T (110), 1 (110), x (101), y (201), n (021), and o (111). 

 The faces n and y are dull, but the others are quite bright. Between 

 y and P there are two other domes on one crystal, of which one is doubt- 

 less X. 



It is probable that both albite and natron-orthoclase are present, and 

 some quantitative chemical tests corroborated this view, although they 

 were not complete enough for an analysis. 



In a large crystal, partly microcline and partly orthoclase, from a peg- 

 matite vein on Masons island, Mystic, Connecticut, weathered surfaces 

 were found running roughly parallel with a somewhat imperfect cleavage, 

 which is itself parallel with the prism faces. The weathered surface was 

 thickly set with little crystals of secondary feldspar whose vertical axis, 

 basal pinacoid, and prism faces are parallel with those of the host. 'J'he 

 former are white, while the" latter is red. The former are striated on the 

 base. As a maximum they are 1.5 millimeters in length and vary from 

 this down to extreme minuteness. The twinning which they exhibit in 

 thin sections parallel with the base makes it evident that they are a pla- 

 gioclase near albite. These secondary feldspars are clearly infiltration 

 products that have come in along the prismatic cleavage cracks, and es- 

 pecially the ones parallel with the right-hand prism face. One can not 

 well say, however, at what stage in the history of the rock they entered, 



